1. Field of the Invention
The present invention relates to the field of anti-skid devices for vehicle wheels and, more particularly, to a unit of the snow-chain type adapted to be automatically positioned on the periphery of the tire upon advancement of the vehicle.
2. Description of the Prior Art
Such a system is known from Swiss Patent 657.812 which describes the basic principle of an anti-skid device arrangement comprising one or more chains which are borne at one end of radial arms and the other end of which is applied along the axis of the wheel by elastic traction devices arranged on the outer face of the wheel.
In Applicant's EP Patent A-312.636, there is described more particularly a type of central housing having, on the one hand, the radial arms bearing the chains and, on the other hand, means for pulling in the direction of the axis of the wheel. European Patent A-351.362 describes an anti-skid travel path which permits the automatic tensioning of the chains on the periphery of the tire.
In these different patents, the system for pulling the unit in the direction of the axis of the wheel is established in all cases by elastic means fixed at at least three points on the rim, in order to hold the central housing on the axis of the wheel and pull it in the direction thereof, so as to maintain the snow chain on the periphery of the tire.
In order to simplify the mounting, it has been attempted to effect a single attachment which permits the mounting, removable at will by the user, of an anti-skid device such as described above. In European Patents EP-A-376.426 and EP-A-376.428 a system is described for mounting on one of the fastening nuts of the wheel by means of a connecting part intended to cooperate with the central housing. It is necessary to tension this spring which is intended to apply and maintain the device in operating position. The drawbacks of such a system are that, after having fastened the connecting part to the wheel of the vehicle, the user must still effect the tensioning of the elastic means. Furthermore, tests have shown that the rotation between the connecting part and the point of attachment is detrimental to the proper operation of the anti-skid device, while the turning of the snow chain, and therefore of the housing, with respect to the axis of rotation of the wheel is indispensable.